The carrier lifetime τ plays an important role in the global effectiveness of solar photovoltaic cells, as it limits the proportion of photo-generated charges collected on electrodes. As such, it is desirable to have substantial carrier lifetimes in solar cells in order to maximise the quantity of charges collected on electrodes. The carrier lifetime τ, also called carrier recombination time or “photo carrier lifetime”, is defined as the duration during which an electron excited by light remains in the conduction band before falling back into the valence band. The carrier lifetime must not be confused with the carrier generation lifetime, which is the duration required to convey an electron in the conduction band.
Prior art knows many methods for determining the carrier lifetime. As such, the document Coffey D, Ginger D, Time-resolved electrostatic force microscopy of polymer solar cells, 2006 Nature materials, 5, 735 propose to illuminate with a light source the surface of the sample of which it is desired to determine the carrier lifetime and to measure the decay of the electric signal obtained once this light source has been extinguished. Document Takihara Appl. Phys. Lett. 93, 021902 2008 proposes to illuminate the surface of the sample with a modulated light then to measure the surface potential via a Kelvin probe force microscopy technique.
However, the techniques of prior art make it possible only to obtain values of the carrier lifetime averaged over a relatively large surface of the sample. As such, none of the techniques of prior art makes it possible to map the carrier lifetime with good lateral resolution. Furthermore, the methods of prior art have a limited temporal resolution.